The test that indicated a dead radio should send up some large red flags!!!
That is not at all an interference problem, it's a complete loss of signal or power problem, unless the radio went into a prolonged failsafe.

What kind of a range check do you get with the engine not running? IMO, 30 or 40 paces with the engine running is not a very good range check. With several different radios, I always get over 180 feet, or more, engine running or not. You should too.

What kind of engine?

Next, what kind of plug?

Is it a resistor plug if one is available for you engine?

Are you using a bunch of old servo extensions that you have had lying around for a long time?

Are the switches new and heavy duty?

Do you have the ignition properly grounded?

Have you tried the range check with only one receiver, instead of both at the same time?

Are the crystals in the receivers held tightly in place with tape?

Are the crystals new, or in good shape, or have they been through a crash before?

Have you installed some foam around the receiver to prevent vibration?

Are all the servo connections tight?

Do you have a bunch of metal to metal connections?

Are the Rx antennas wrapped around the servo leads?

Do you have good battery connections?

I would look at all of the above before I started to move anything around. It's likely that the problem is not from location, but from loose or worn parts, or improper installation practices.